Nuclear fuel rods (also referred to as nuclear fuel assemblies) frequently require movement and transportation, for example to enable refueling of reactor cores or to take fuel rods for processing, storage or disposal.
Manipulators having gripper mechanisms are used to move and transport the fuel rods. The gripper mechanisms need to reliably attach onto the fuel rods so as to facilitate their safe transfer. Due to the hazardous nature of nuclear fuel rods, these gripper mechanisms should meet International Atomic Energy Agency (IAEA) recommendations, including:                (a) Obtaining a positive indication that the gripper mechanism is correctly located on the fuel rod before lifting is commenced;        (b) Once a fuel rod is gripped, the gripper mechanism should remain latched to a fuel rod upon loss of power;        (c) The gripper mechanism should not be capable of decoupling from the fuel rod while a load is applied;        (d) The gripper mechanism should only decouple from its load at specified elevations, even when no load is applied; and        (e) The gripper mechanism should have an inherent safety device that prevents the fuel rod from becoming undesirably released from the gripper mechanism.        
Current designs of gripper mechanisms are generally mechanically controlled rather than electrically controlled because radioactive environments tend to damage electrical equipment. It is also generally important to keep gripper mechanism design simple, as added complexity typically creates more risk.
Currently available gripper mechanisms grip the fuel rods either externally or internally via a cavity formed in a plug end of the fuel rod. Gripper mechanisms that grip the fuel rods externally interfere more with the immediate environment than gripper mechanisms that grip the fuel rods internally. As a result, when the fuel rod is gripped internally there is a reduced risk of collision with other surrounding nuclear fuel rods and of gripping onto and transferring something not intended for transportation.
An example of a known gripper mechanism that grips fuel rods internally is disclosed in U.S. Pat. No. 4,747,997A. The gripper mechanism of U.S. Pat. No. 4,747,997A operates by inserting inwardly biased fingers, with outwardly biased extensions, into a cavity formed in the plug end of a fuel rod. The biased fingers are then splayed open by an inner shaft. A circumferential groove is formed in the cavity of the fuel rod, and the biased fingers, when splayed, engage the circumferential groove of the cavity.
Gripper mechanisms having an internal engagement (similar to that described in U.S. Pat. No. 4,747,997A) often suffer from a difficulty in ascertaining whether or not the gripping mechanism has correctly engaged the fuel assembly (which is a recommendation of the IAEA standard).
Gripper mechanisms having an alternative configuration are also known. For example, many known gripper mechanisms include a latching mechanism that is activated by interaction with an external feature (e.g. a ring that surrounds an access entry point to the fuel rod).